The heart is the center of a person's circulatory system. It includes an electro-mechanical system performing two major pumping functions. The left portions of the heart draw oxygenated blood from the lungs and pump it to the organs of the body to provide the organs with their metabolic needs for oxygen. The right portions of the heart draw deoxygenated blood from the organs and pump it into the lungs where the blood gets oxygenated. In a normal heart, the sinoatrial node, the heart's natural pacemaker, generates electrical impulses, known as action potentials, that propagate through an electrical conduction system to various regions of the heart to cause the depolarization of the electrical conduction system and excitation of myocardial tissues in these regions. Coordinated delays in the propagations of the electrical impulses in a normal electrical conduction system cause the various regions of the heart to contract in synchrony such that the pumping functions are performed efficiently. Arrhythmia occurs, for example, when the sinoatrial node fails to generate the electrical impulses at a normal rate, when electrical impulses are generated from a pathological origin, and/or when pathological changes occur to the electrical conduction system. Arrhythmia causes the heart to contract at a rhythm that is too slow, too fast, or irregular. Consequently, the heart's pumping efficiency is reduced, and hence, the blood flow to the body is diminished.
Implantable CRM devices are used to treat arrhythmias by delivering electrical pulses to the patient's heart. In one example, pacing pulses are delivered to one or more regions of the heart to at least partially restore the function of the sinoatrial node and/or the electrical conduction system. According to many pacing algorithms, a pacing pulse is delivered on demand, i.e., when a corresponding intrinsic depolarization is absent or abnormally delayed. In another example, a defibrillation pulse is delivered to the heart to stop a rhythm that is too fast and/or irregular. This requires detection of a depolarization rate and/or pattern that warrant a delivery of the defibrillation pulse. Thus, the detection of cardiac electrical events including depolarizations is important in both pacing and defibrillation therapies.
The cardiac depolarizations are detected from one or more cardiac signals each sensed with at least one electrode placed in or on the heart. In addition to cardiac depolarizations, noises of various types are often present in such cardiac signals. The sources of such noises include, but not limited to, non-cardiac bioelectric activities such as myoelectrical signals associated with breathing and/or bodily movements and interference from nearby electrical power lines, equipment, and appliances. An implantable CRM device detects a cardiac depolarization when the amplitude of a cardiac signal exceeds a detection threshold. When the threshold is set low, the noises may cause over-sensing, i.e., the implantable CRM device detects noise as cardiac depolarizations. Consequently, the implantable CRM device fails to deliver pacing pulses when needed and/or delivers a defibrillation pulse that is not needed. When the threshold is set high to avoid detection of noise, under-sensing may occur, i.e., the implantable CRM device fails to detect cardiac depolarizations. Consequently, the implantable CRM device delivers of pacing pulses that are not needed or not properly timed based to the heart's intrinsic activities and/or fails to deliver a defibrillation pulse when fibrillation occurs. Depending on the type of therapy, over-sensing and under-sensing both have consequences ranging from inefficient therapy to death. For example, the consequence of a failure to deliver a defibrillation pulse may be fatal, while the consequence of a delivering an unnecessary defibrillation pulse causes significant discomfort to the patient and shortens the life expectancy of the implantable CRM device. For these and other reasons, there is a need to provide an acceptably accurate detection of cardiac electrical events in the presence of noise.